C++ named requirements: EqualityComparable

 

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C++ named requirements

Basic
DefaultConstructible CopyAssignable CopyConstructible	MoveAssignable (C++11) MoveConstructible (C++11) Destructible
Type properties
ScalarType TriviallyCopyable (C++11) StandardLayoutType (C++11)	PODType TrivialType (C++11) ImplicitLifetimeType (C++20)
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EqualityComparable LessThanComparable Swappable (C++11) ValueSwappable (C++11) NullablePointer (C++11) Hash (C++11)	Allocator FunctionObject Callable Predicate BinaryPredicate Compare
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The type must work with == operator and the result should have standard semantics.

Requirements

The type T satisfies EqualityComparable if

Given

• a, b, and c, expressions of type T or const T

The following expressions must be valid and have their specified effects

Expression	Return type	Requirements
a == b	implicitly convertible to bool	Establishes an equivalence relation, that is, it satisfies the following properties: For all values of a, a == a yields true. If a == b, then b == a If a == b and b == c, then a == c

Notes

To satisfy this requirement, types that do not have built-in comparison operators have to provide a user-defined operator==.

For the types that are both EqualityComparable and LessThanComparable, the C++ standard library makes a distinction between equality, which is the value of the expression a == b and equivalence, which is the value of the expression !(a < b) && !(b < a).

See also

equality_comparableequality_comparable_with (C++20)

specifies that operator == is an equivalence relation (concept)